In an optical disk drive, the tracking servo control and the focus servo control are performed so as to make a light beam spot follow the surface deflection of an optical disk and the runout of a track. The position of a beam spot is controlled so that a tracking error signal becomes zero in the tracking servo system and a focus error signal becomes zero in the focus servo system.
At present, in optical disk drives, the disk for storing data can be removed and another disk loaded. For this reason, the optimum parameters of the servo system are not always constant. In addition, since disk characteristics are not uniform, the drive should perform a self-calibration so that the parameters of the servo system are optimized.
Tracking error signals and focus error signals are calibrated each time a disk is loaded so that reading, writing, and access can be performed correctly even in cases where disk characteristics are not uniform. Since there are cases where characteristics are not uniform even on one disk, it has been typical to perform calibration at several places in one disk. Calibration has not been convenient because, after a disk was loaded, it took several seconds in some cases before it was possible to read and write data.
Several calibration methods have been proposed.
First, a method disclosed in "86 mm Magneto-optical Disk Drive," Nakashima et al, SPIE Vol. 1316, Optical Data Storage, pp. 16-29 (1990) is explained. The amplitude (TEp-p)det of a tracking error signal to be generated by disk runout with the tracking servo OFF and the focus servo ON each time a disk is loaded is detected and it is compared with a desired value (TEp-p)typ so as to obtain the specification coefficient Cte=(TEp-p)typ/(TEp-p)det. Next, the signal level of pre-pit to be reproduced by adding an offset to the tracking error signal is checked and positive and negative offset values in levels a level lower than the maximum value are taken as OFFSET1 and OFFSET2 and the center value is taken as OFFSETte, a detection offset. With these, the calibrated tracking error signal TEcal can be calculated by TEcal=Cte * Cagc , (TE-OFFSETte). Here, Cagc is a compensation coefficient because the signal level of pre-pit greatly depends on the laser intensity. For the focus error signal, the amplitude of an S-shaped curve is measured by moving the objective lens up and down each time a disk is loaded and treatment similar to that of the tracking error signal is performed.
However, this procedure takes time because the amplitude of the tracking error signal cannot be detected unless the tracking servo is turned off first. There is an additional drawback in that the response is delayed when a request for reading and writing data is made even if calibration is performed while the data reading or writing operation is not being carried out. In addition, if noise due to defects and dispersion of a disk is present during amplitude detection, it appears as a calibration error. That is, this literature does not show an effective noise removal method.
A noise removal method is proposed in "Drive Control Technology in 90 mm Optical Disk Drive," Yoshimoto et al, Mitsubishi Electric's Technological Report, Vol. 66, No. 6, pp. 629-633 (1992). In this reference, a method is described for removing the influence of noise and compensating an offset of the tracking error signal by repeating track jumps many times in one rotation of a disk and measuring a maximum value and a minimum value of the tracking error signal and averaging them. However, there have been problems in that the measurement of an offset value to be canceled is carried out at the point when a disk is loaded, and it takes several seconds until it is possible to read and write data. Furthermore, it also has had a problem similar to that of the first reference in that the offset change with the time passage could not be treated. In addition, this literature does not show a calibration method regarding the amplitude of the tracking error signal and the offset of the focus error signal.
In PUPA No. 4-278233 gazette, the offset value is calculated in accordance with a maximum value and a minimum value of a tracking error signal generated during a seek operation and a track jump as a method for canceling an offset included in the tracking error signal of an optical disk drive. For the calculation method, a maximum value and a minimum value of the tracking error signal are held in a sample hold circuit at the timing of positive peak and negative peak, respectively, and their mean value is stored as an offset value or added directly to the tracking error signal. In the former case, the track address is also detected and stored by making it correspond to the offset value. The stored value is read at a later time and the offset is canceled in the same method as the latter case.
However, in this method, a maximum value and a minimum value of an improper tracking error signal may be detected when a defect is generated on a disk. There is, therefore, a possibility of setting the offset value of the tracking error signal to an improper value.
PUPA No. 4-232624 gazette discloses a method for acquiring an optimum focus by repeating measurements of the amplitude of the tracking error signal until the difference between the two tracking error signals obtained becomes small enough in a state where positive and negative increments are added to the focus error signal, and for determining a compensation value of the focus error signal.
However, it is also difficult to deal with the time fluctuation the offset of the focus error signal in this method. In addition, it takes time because the difference of the tracking error signal converges.
None of the above references disclose a method for performing calibration of the tracking error signal and the calibration of the focus error signal in a unified manner. Therefore, the calibration of those signals has to be performed separately by different methods, each contributing to the total time required for calibration.
In addition, none of the above references disclose the use of a tracking error signal generated upon a track jump in the track following mode for the calibration of the tracking error signal and the focus error signal.